1. Field of the Invention
The present invention relates to an image processing apparatus, such as a copying machine or a multifunction peripheral, capable of performing a scanning function and a printing function, and more particularly to an image processing apparatus which is capable of processing a scanned image and identifying a large dot region and a small dot region.
2. Description of the Related Art
Various techniques for printing copy-forgery-inhibited pattern images are known, one of which is discussed in Japanese Patent Application Laid-Open No. 2004-228896. Techniques for processing and printing copy-forgery-inhibited pattern images are described below.
FIG. 1 illustrates an original image 5001. FIG. 2 illustrates that a print product 5010 can be obtained by printing a composite image that is generated by combining the original image 5001 with a copy-forgery-inhibited pattern image 5002.
When the print product 5010 is copied, a copy product 5001′ illustrated in FIG. 4 is obtained. Thus, when the print product 5010 obtained by combining the original image 5001 with the copy-forgery-inhibited pattern image 5002 is copied, a latent image 4002, which has been hidden in the print product 5010, emerges on the copy product 5001′.
Thus, even when a print product obtained by combining an original image with a copy-forgery-inhibited pattern image is copied, users cannot obtain a copy product that is exactly the same as the print product. The copy-forgery-inhibited pattern image is directed to a technique for preventing a user from obtaining a copy product that is exactly the same as a print product.
Copy-forgery-inhibited pattern images employed to prevent a user from obtaining a copy product that is exactly the same as a print product are frequently used in a case where a print product (e.g., a copy of a residence card, or an insurance certificate), the assurance of authenticity of which is demanded, is produced.
Next, it is described below how a copy-forgery-inhibited pattern image is generated.
The copy-forgery-inhibited pattern image includes dot groups that differ in dot size from each another. An example of a copy-forgery-inhibited pattern image illustrated in FIG. 3 includes a small dot group 5002a and a large dot group 5002b. An image of a region, in which the large dot group 5002b is located, is referred to as a latent image. An image of a region, in which the small dot group 5002a is located, is referred to as a background image.
The small dot group and the large dot group are substantially equal in density per given area to each other on a print product. Therefore, even when a user glances the print product, the user cannot distinguish the latent image from the background image. Accordingly, the latent image is not seen to emerge from the background image.
On the other hand, as described above, when a copy-forgery-inhibited pattern image is copied, a latent image emerges from a background image. This is because of the fact that a copying machine has ability to exactly reproduce large dots on a print product, but it lacks ability to exactly reproduce small dots on the print document.
To explain this phenomenon in more detail, first, a configuration of a copying machine is described below.
As illustrated in FIG. 5, a copying machine 6000 includes an image reading device 6001. The image reading device 6001 (e.g., a scanner or a digital camera) reads a surface of a print product using an optical element 6002, typified by a charge coupled device (CCD), to generate a scan image as electrical signals. The scan image is sent to an image processing apparatus 6003. Then, the image processing apparatus 6003 performs scanner image processing on the scan image. In the context of the present specification, an image obtained by performing scanner image processing on the scan image is referred to as a read image. Subsequently, the image processing apparatus 6003 temporarily stores the read image in a storage unit 6005. Thereafter, the image processing apparatus 6003 reads the stored image and performs printer image processing on the read image. Then, the image processing apparatus 6003 outputs the processed image to a printer 6004. Finally, the printer 6004 receives the processed image and prints the processed image on a sheet.
FIG. 6 illustrates that when a copy-forgery-inhibited pattern image 1600 formed on a print product is copied by the copying machine 6000, a copy product 1603 is obtained.
A part of the latent image 1601 of the copy-forgery-inhibited pattern image 1600 is a large dot region. A part of the background image 1602 of the copy-forgery-inhibited pattern image 1600 is a small dot region. A size of each small dot is about 42 μm×42 μm (corresponding to 1 pixel at a resolution of 600 dpi (dots per inch)). A size of each large dot is about 126 μm×126 μm (corresponding to 3×3 pixels at a resolution of 600 dpi).
When such large dots are read by a CCD having a resolution of 600 dpi, an image 1606 in a state in which each pixel is dark (i.e., has a low luminance value) is generated. Conversely, when small dots are read by such a CCD, an image 1607 in a state in which each pixel is light (i.e., has a high luminance value) is generated.
When a scan image including the images 1606 and 1607 is sent to the image processing apparatus 6003, the image processing apparatus 6003 performs scanner image processing on the scan image. The scanner image processing includes a background color removal process, a logarithm conversion process, and a gamma correction process.
Among these processes, the background color removal process may be the most principal cause of a phenomenon that a latent image emerges from a background image on a copy product. The background color removal process aims to eliminate, for example, a light gray part of a surface of a sheet of newspaper and to convert a light color pixel into a white or lighter-color pixel.
A light-color image 1607 obtained by the CCD reading small dots is converted into a lighter-color image 1609 by the background color removal process. On the other hand, a dark image 1606 obtained by the CCD reading large dots remains as a dark image 1608.
That is a reason for the phenomenon that when an copy-forgery-inhibited pattern image is copied by a copying machine having a resolution of 600 dpi, a latent image emerges from a background on a copy product.
However, nowadays, image reading devices each including a CCD, whose resolution exceeds 600 dpi, have become increasingly popular. In a case where a copy-forgery-inhibited pattern image is read by such an image reading device, even when a background color removal process is performed on a background image thereafter, the probability of occurrence of a phenomenon in which the color of the background image becomes lighter or white decreases.
This phenomenon is explained below with reference to FIG. 7. FIG. 7 illustrates that when a copy-forgery-inhibited pattern image 1700 formed on a print product is copied, a copy product 1703 is obtained.
For the sake of explanation, suppose that the resolution of the CCD illustrated in FIG. 7 is 1200 dpi, which is twice that in the case illustrated in FIG. 6. Accordingly, when small dots are read by the CCD, an image 1707 is generated, in which a part of pixels of the image 1707 are dark (i.e., have low luminance values). Even when a background color removal process is performed on the image 1707 having such dark pixels, the dark pixels are not converted. That is, the dark pixels remain dark. Thus, an image 1709 is generated. Therefore, even when a copying process is performed on a copy-forgery-inhibited pattern image formed on a print product, the color of a background image does not become so light. Accordingly, a latent image may not emerge from the background image as intended.
Consequently, a copy product is formed such that the copy product may be nearly the same as the print product (i.e., an original) including a copy-forgery-inhibited pattern image. Accordingly, the copy product may be indistinguishable from the original.